As mobile technology evolves, mobile devices often need to tune to multiple bands in order to successfully receive signals. The radio frequency (RF) front end of a mobile device may require many fixed bands to be supported and needs to tune to these bands.
Many mobile devices use surface acoustic wave (SAW) filters in order to proper tune to a band. A SAW filter permits only the signal in the band being filtered and removes all other interference. However, a SAW filter is not tunable and a device required to receive signals on multiple bands will need multiple SAW filters. As devices require more and more bands for reception, this becomes problematic for both the cost and the space required within the mobile device.
Radio frequency integrated circuits generally cannot handle or remove interference out-of-band without off-chip filtering. A tunable duplexer allows tuning to a desired band, but may have very light filtering of the receive (RX) band compared to a SAW filter. For example, a second order band pass filter with a Q of 15 has as little as 5 dB band edge rejection. Also, because a duplexer will tune from one band to another, the RX rejection in nearby bands will not be good enough since it is very difficult to tune into one band and then reject the entire band in a future tuning.